Water and sunlight: a winning catalytic combination

Researchers have incorporated a sunlight-activated trigger into an oxygenation process that uses water as the oxygen source. The combination approach is a step towards mimicking nature’s photocatalytic processes and could help reduce carbon dioxide emissions and lead to further applications for solar energy. 

Shunichi Fukuzumi and coworkers from Osaka University in Japan and Wonwoo Nam from Ewha Womans University in Seoul, Korea and colleagues have developed highly efficient photocatalytic oxygenations of organic substrates such as sodium p-styrene sulfonate, using sunlight as an initiator and water as an oxygen source. 

The team employed a ruthenium (II) complex that absorbs light and uses the energy to reduce a cobalt complex. The ruthenium (III) complex then goes on to react with a manganese porphyrin oxygenation catalyst. This reaction causes the manganese porphyrin complex to give off a proton, and become the active species responsible for the oxidation of organic substrates. The catalyst can oxygenate substrates by using water in a phosphate buffer solution (pH 7.4) as an oxygen source.

’This is an important step for the general use of water as a clean and abundant reactant using solar energy,’ say Fukuzumi. He goes onto explain that current industrial oxygenation processes are energy consuming, resulting in large amounts of CO₂ emissions. ’Oxygenation reactions using water as an oxygen source under mild conditions using solar energy would contribute to cut down CO₂ emissions,’ he adds.

Richard Douthwaite, an expert in photocatalysis from the University of York in the UK, believes that the work is an interesting proof of principle. ’What they appear to have done is take two known reactions and stitch them together,’ he says. ’But no-one has shown before that light can be used to mediate these oxidation processes with water,’ he adds.

Fukuzumi acknowledges that there is still work to do to make the process truly environmentally benign. The team’s aim is to replace the cobalt complex with molecular oxygen ’to develop oxygenation reactions with water as the oxygen source, oxygen as the oxidant, under mild conditions using solar energy.’

Mike Brown